1. Field of the Invention
The present invention relates to a sheet stacking apparatus which stacks sheets, a sheet processing apparatus which processes sheets stacked on the sheet stacking apparatus, and an image forming apparatus having these apparatuses.
2. Description of the Related Art
Conventionally, as an image forming apparatus which forms an image on a sheet, there is an apparatus which has a sheet processing apparatus. An apparatus main body forms images on sheets, and the sheet processing apparatus puts sheets together into a sheet bundle, binds them and folds them into a booklet. The sheet processing apparatus sequentially receives sheets on a tray, puts the sheets together into a sheet bundle, aligns the same, binds their central portions, pushes the central portions with an pushing member and pushes them into nips of a pair of fold rollers, the pair of fold rollers convey a sheet bundle and fold the sheets (see Japanese Patent Application Laid-Open No. 2007-076793).
The operation of such a conventional sheet processing apparatus will be described based on FIG. 16. As illustrated in FIG. 16, the sheet processing apparatus first aligns a plurality of sheets by a storing guide 803, and binds central portion thereof in the conveying direction using a staple. Then, a pushing member 830 pushes the central portions of the sheet bundle P, and pushes them into a nip of first pair of fold rollers 810a and 810b. The first pair of fold rollers 810a and 810b convey the sheet bundle and in this state, the first pair of fold rollers 810a and 810b fold the sheets and once stop.
The folded portions of the sheets are nipped by a second pair of fold rollers 812a and 812b which are different from the first pair of fold rollers 810a and 810b, and move the second pair of fold rollers 812a and 812b in a direction intersecting with the conveying direction along a crease of the sheets, thereby reinforcing the folding portion. Accordingly, the sheet bundle becomes a center-folded sheet bundle (simply “folded sheet bundle”, hereinafter). Then, the folded sheet bundle are conveyed and discharged into a fold bundle tray 840.
As the sheet stacking apparatus, there is proposed an apparatus having a rear end pressure member which prevents a rear end of a stacked sheet from floating from a stacking portion (see Japanese Patent Application Laid-Open No. 2006-306522).
In recent years, image quality level of image forming apparatuses is enhanced, and sheets on which images are formed are diversified. For example, an image can be printed on a special sheet whose surface is worked such as a coated paper, and on a wide grammage (thin and weak paper or thick and hard paper).
However, in a structure in which sheets are aligned by the storing guide 803 whose downstream side in a discharging direction is inclined downward, if the sheets are weak, the sheets (bundle) are folded by their own weight or air is stored in the sheet bundle and as a result, sheets swell in some cases. Not only rear ends of sheets, but also central portions swell in some cases, and this affects the subsequent conveying performance of sheets, and in an apparatus which aligns the sheets, alignment failure may occur.
For example, as illustrated in FIG. 17A, if a rear end area (upstream end in the discharging direction) of a sheet P1 which swelled is pushed against a stacking surface by a pressure member 16 from a vertical direction, the swelling portion near the rear end of the sheet P1 adversely moves to the central portion as illustrated in FIG. 17B. If such a situation occurs, in a structure in which a rear end of the stacked sheet P1 is held by a holding member 11, a sheet P2 which is to be conveyed next can not be conveyed (state illustrated in FIG. 18B) due to a holding failure (state illustrated in FIG. 18A) or the swell in some cases.
To solve this problem, it seems to be a good idea to bring a stacking portion of a sheet into substantially a horizontal attitude so that the apparatus does not receive the influence of its own weight, but the apparatus is increased in size in the lateral direction, and the installation area of the apparatus is increased.
If a plurality of assortment portions (switching members or the like) are provided, sizes of sheets which are conveyed are limited to a predetermined size only.
As illustrated in FIG. 19A, since the position of the switching member 802 is limited, a rear end portion of the stacked sheet P1 and a tip end portion of a sheet P2 which is conveyed next collide against each other and a jam is generated depending upon a size of the sheets.
As illustrated in FIG. 19B, a rear end of a sheet leans on a tip end 802a1 of the switching member 802 and this causes the conveying failure.
The present invention provides a sheet stacking apparatus, a sheet processing apparatus and an image forming apparatus capable of stacking and processing various sheets such as weak sheets without causing conveying failure and alignment failure.